1. Field of the Invention
The present invention relates to a process for the crystallization of an inorganic substance, using a bed of crystals through which passes a solution which is supersaturated with the substance to be crystallized.
2. Description of the Related Art
The xe2x80x9cOsloxe2x80x9d crystallizer is a well-known apparatus for the crystallization of inorganic substances (British Chemical Engineering, August 1971, Vol. 16, No. 8, pages 681 to 685; The Chemical Engineer, July/August 1974, pages 443 to 445; British Patent GB-A-418,349). This known apparatus comprises a vertical cylindrical vessel and a vertical tube which is arranged axially in the vessel and which opens in the immediate vicinity of the bottom of the latter; a vertical annular chamber is thus defined between the axial tube and the cylindrical wall of the vessel. In making use of this known apparatus, a bed of crystals is employed in the annular chamber, through which passes a solution supersaturated with the substance which it is desired to crystallize (for example an aqueous solution supersaturated with sodium chloride). This solution is introduced into the apparatus via the axial tube, so that it enters the annular chamber radially, near the bottom of the latter, and subjects the crystals in the bed to a general rotation comprising an upward translation along the wall of the vessel and a downward translation along the axial tube.
In this known apparatus the intention is to produce uniform crystalline particles of spherical shape, whose mean diameter it should be possible to control by an appropriate choice of the dimensions of the apparatus and operating conditions. In practice, however, this known apparatus is ill-suited for obtaining large spherical particles, especially because of the attrition produced within the bed of crystals and of the general rotation to which these are subjected. In particular, in the case of sodium chloride, it does not permit spherical particles greater than 2 or 3 mm in diameter to be produced. Furthermore, an effect of the attrition phenomenon is to give rise to the formation of fines which are entrained out of the bed by the crystallization mother liquor and which must consequently be separated from the mother liquor before the latter is recycled into the apparatus.
The invention is aimed at overcoming this disadvantage, by providing a new process which makes it possible, on the one hand, to crystallize particles which are spherical and of larger diameter and, on the other hand, to reduce the formation of fines.
Consequently, the invention relates to a process for the crystallization of an inorganic substance, in which use is made of a bed of crystals through which passes a stream of a solution supersaturated with the substance to be crystallized, the bed of crystals being fluidized by passing the supersaturated solution through a distributor which is arranged below the bed of crystals and which is maintained at a temperature at which the concentration of the supersaturated solution does not exceed the concentration corresponding to saturation.
In the process according to the invention, the crystals in the bed act as seeds for the crystallization of the inorganic material by desupersaturation of the supersaturated solution. They are generally small uniform crystals of the inorganic substance which it is intended to crystallize.
The degree of supersaturation of the supersaturated solution depends on various parameters, especially on the nature of the inorganic material, on its temperature and on the possible presence of solid or dissolved impurities. In practice, everything else being equal, it is advantageous to produce a maximum degree of supersaturation, although this must be limited to avoid accidental crystallization on the walls of the crystallization plant, upstream of the crystal bed, as well as primary and secondary seeding within the solution.
The means employed to obtain the supersaturated solution are not critical. The latter may be obtained, for example, by changing the temperature or by partially evaporating a solution saturated beforehand with the substance to be crystallized.
The solvent in the solution is not critical, and water is generally preferred.
The temperature of the supersaturated solution is not critical. In practice, however, it has been observed that the rate of growth of the crystals in the bed is proportionally greater the higher the solution temperature. It is advisable, however, that the solution temperature should remain below its boiling point at the pressure prevailing in the crystallization chamber. For example, in the case where the process is applied to the crystallization of sodium chloride, it is possible advantageously to employ aqueous solutions of sodium chloride with a degree of supersaturation between 0.3 and 0.5 g/kg, at a temperature of between 50 and 110xc2x0 C. The degree of supersaturation expresses the excess mass of inorganic substance relative to the mass corresponding to the saturation of the solution.
According to the invention, the crystal bed is a fluidized bed, in accordance with the generally accepted definition (Givaudon, Massot et Bensimonxe2x80x94xe2x80x9cPrxc3xa9cis de gxc3xa9nie chimiquexe2x80x9d (A summary of chemical engineering)xe2x80x94volume 1xe2x80x94Berger-Levrault, Nancyxe2x80x941960, pages 353 to 370). To fluidize the bed, the stream of the supersaturated solution is passed through a distributor arranged under the crystal bed, in accordance with the usual technology of fluidized bed reactors. The distributor is a fundamental member of the fluidized bed reactors. Its function is to divide the stream of solution into thin jets, preferably parallel and vertical, while furthermore imposing onto it a defined pressure drop, controlled as a function of the bed dimensions, of the nature of the particles forming the bed and of the solution (Ind. Eng. Chem. Fundam.xe2x80x941980xe2x80x9419xe2x80x94G. P. Agarwal and othersxe2x80x94xe2x80x9cFluid mechanical description of fluidized beds. Experimental investigation of convective instabilities in bounded bedsxe2x80x9dxe2x80x94pages 59 to 66; John H. Perryxe2x80x94Chemical Engineers"" Handbookxe2x80x944th editionxe2x80x941963xe2x80x94McGraw-Hill Book Companyxe2x80x94pages 20.43 to 20.46). It may be, for example, a horizontal plate pierced with uniformly spaced orifices, a grid or a horizontal mesh, or an assembly of vertical nozzles.
In accordance with the invention, the distributor is maintained at a uniform temperature at which the concentration of the supersaturated solution is lower than or equal to the concentration corresponding to saturation. In other words, in the process according to the invention, the temperature of the distributor is different from the temperature of the supersaturated solution upstream of the distributor, and it is chosen so as to make the concentration of the said supersaturated solution lower than or at most equal to the concentration of the saturated solution which is stable at the temperature of the distributor (for the same single inorganic substance and the same single solvent as the supersaturated solution). The crystallization of the inorganic substance on the distributor is thus avoided. The choice of the distributor temperature is critical and depends on the inorganic substance which it is intended to crystallize, the solvent in the solution and the degree of supersaturation. Thus, in the case of a substance whose solubility in the solvent increases with temperature (for example an aqueous solution of sodium or potassium chloride) the temperature must be higher than that of the supersaturated solution. In the case of a substance whose solubility in the solvent varies inversely with temperature (for example an aqueous solution of sodium carbonate monohydrate), the temperature must be lower than that of the supersaturated solution. Furthermore, the choice of the distributor temperature is conditioned by the need to prevent the supersaturated solution coming into contact with it from undergoing an excessive temperature change, which would result in an exaggerated drop in its degree of supersaturation. The temperature of the distributor must consequently be controlled to make the stream of solution in the distributor undergo a temperature change which is insufficient to desupersaturate it completely. In practice, the distributor temperature must be determined in each particular case, especially as a function of the degree of supersaturation of the solution and of the required production efficiency. Its choice will also depend on various factors such as the heat transmission coefficient of the distributor, the flow rate and the temperature of the solution, and its specific heat. In each particular case, the definition of the optimum temperature of the distributor can be determined without difficulty by calculation and experiment.
A substantially saturated mother liquor is collected downstream of the crystal bed. This can be recycled into the process after it has been treated to bring it into a supersaturated state. For this purpose, in a particular embodiment of the process according to the invention, the mother liquor collected downstream of the crystal bed is heated, and a complementary quantity of the inorganic substance which is equal to that which has crystallized in the bed is added to it, and the resulting mixture is then cooled to reconstitute the supersaturated solution. This embodiment of the process applies specifically to the inorganic substances whose solubility in the solvent varies in the same direction as the temperature. The heating of the mother liquor must be sufficient for all the abovementioned complementary quantity of inorganic substance to dissolve therein. The cooling of the mixture can be obtained by circulating it through a heat exchanger or by subjecting it to a decompression, to evaporate it partially. When the cooling is performed by decompression, a complement of solvent should be added to the mother liquor to compensate the quantity evaporated by the decompression.
In a second particular embodiment of the process, applied specifically to the inorganic substances whose solubility in the solvent varies inversely with the temperature, the mother liquor collected downstream of the bed is cooled, a complementary quantity of the inorganic substance which is equal to that which has crystallized is added to it, and it is then heated to reconstitute the supersaturated solution.
The invention also relates to a plant for the cystallization of an inorganic substance according to the process in accordance with the invention, comprising a vertical tubular enclosure, a vertical tube arranged axially in the enclosure and opening in the vicinity of its bottom, so as to define an annular chamber in the enclosure, the tube being joined at its upper end to a device for feeding a solution which is supersatured with the substance to be crystallized; a distributor divides the annular chamber into a lower entry chamber for the supersaturated solution entry and an upper crystallization chamber forming a fluidized bed reactor, the distributor being provided with a thermostat.
In the plant according to the invention, the function of the thermostat is to maintain the distributor at a uniform temperature, controlled as a function of that of the supersaturated solution employed, to prevent a spontaneous crystallization of the inorganic substance in contact with the distributor. The thermostat may accordingly comprise a means for heating or a means for cooling the distributor, depending on whether the plant is intended for the treatment of supersaturated solutions of inorganic substances whose solubility is an increasing function or a decreasing function of the temperature.
In an advantageous embodiment of the plant according to the invention, the distributor is made up of vertical or oblique nozzles joining two transverse plates through which they pass, the plates joining the vertical tube and the wall of the tubular enclosure to define a chamber connected to a source of a heat-transfer fluid (for example water or steam) forming the thermostat.
In the process and the plant according to the invention, the fluidized bed performs a dual function. Firstly, it forms a crystallization environment; secondly, it produces a particle size classification of the crystals, which distribute themselves in the bed in layers or strata of equal particle size.
The process and the plant according to the invention allow an inorganic substance to crystallize in the form of uniform particles of approximately spherical shape. These are generally monolithic beads, which means that they are single, unagglomerated blocks of the inorganic substance. The process and the plant according to the invention find an especially advantageous application for the production of sodium chloride crystals in the form of monolithic spherical beads whose diameter is greater than 3 mm, for example between 3 and 30 mm. Crystals of sodium chloride with a diameter of between 5 and 10 mm find an advantageous application for the production of salt in the form of irregular particles and translucent and vitreous in appearance, using the technique described in document EP-A-162,490 (Solvay and Cie).